It would be an understatement to say that computers have changed our society. Computers have made it possible to communicate data around the world in seconds, have access to more information than any one person could assimilate in ten life times, and countless other advantages. And, advances in computer technology are occurring every day. For example, video image processing is making dramatic advances in picture quality for all types of image sources such as multimedia imaging, video conferencing, video games, VCR (video cassette recorder), broadcast television, cable television, and DVD (digital video disks), to name just a few.
Displaying video images that were specifically designed to be displayed on a computer display can be done with the utmost of clarity. For example, still images and computer graphical interface images are displayed with great clarity on computer displays where the display refresh (or update) rate is set by standards bodies which may be 60 Hz, 75 Hz, 90 Hz, or 100 Hz. Because of the static nature of these images, the number being prepared for display, or the image update rate, exactly match the display update rate (the number of images the display will present in a second). Similarly, for television, the incoming images have an image update rate that exactly matches the display update rate of the television. In North America the television display update rate is approximately sixty Hertz and in Europe the television display update is approximately fifty Hertz.
A difficulty arises when displaying images with very precise update rates and were not specifically designed to be displayed on a computer display, such as images from television broadcasts, VCRs, DVDs, and cable television broadcasts. These video transmission mediums have their own image update rates. For example, North American television has an image update rate of 59.94 Hz, while European television has an image update rate of 50 Hz, and motion pictures have an image update rate of 24 Hz. Due to the differences between the image update rate of these video image mediums and the display update rate of the computer display, which are very small, update rate compensation is needed.
One update rate compensation technique is to drop or repeat an image when the timing between the image update rate and the display update rate is offset by an image (i.e., becomes a frame out of sync). When the image update rate is faster than the display update rate, images need to be dropped (i.e., not shown). While this keeps the image update rate in sync with the display upte rate, the dropping of the images may be noticeable. For example, if the image that is dropped is one in a series of action images (an object is moving), there may be a noticeable jump in the movement of the object because of the dropped image. When the image update rate is slower than the display update rate, images need to be repeated to keep the update rates in sync. The repeated image may produce a noticeable jump for objects in motion, similar to the dropped image. Thus, this technique works well for still image projection, but for movies, sporting events, and other television broadcasts, it is not desirable because of the noticeable jumps.
Another update rate compensation technique is to reset the display update rate to equal the image update rate. While this overcomes the above mentioned difficulty, it creates a new one. When the display update rate is coarsely adjusted for some display devices during a mode change, the screen blanks for a few seconds to reset the display update rate. If the display update rate needs to be changed to initiate video, the blanking of the screen can be quite annoying to the user.
Therefore, a need exists for a method and apparatus that finely adjusts the display update rate without the adverse effects mentioned above and does so in a visually acceptable manner.